Recently, an application of a liquid crystal display device to equipment like a personal computer, a TV set, a word processor, a video camera, etc. has been advancing. On the other hand, there has been an increasing demand for such equipment with improved functions including downsizing, power saving, cost reduction, etc. To meet such a demand, a reflective liquid crystal display device which displays an image by reflecting incident light from the external with a reflector instead of using a backlight has been developed as an alternative of a transmission liquid crystal display device which displays an image using light emanated from the backlight.
Further, of all kinds of the reflective liquid crystal display devices, the one adopting an active matrix liquid canasta display pane whose pixels are driven by active elements, such as TFTs (Thin Film Transistors), has been receiving more attention than the one adopting a direct matrix liquid crystal display panel, because an image can be displayed with a better quality at a higher duty.
However, when the power source of the main body of the liquid crystal display device equipped with the active matrix liquid crystal display panel is turned OFF, an image that has been displayed right before the power source is turned OFF remains on the panel as an afterimage for awhile. The afterimage is caused by charges withheld in liquid crystal due to a voltage withheld therein, an abnormal voltage generated by the active elements when the power source is turned OFF, etc. The afterimage degrades the image quality of this kind of liquid crystal display device serving as a display machine.
The transmission liquid crystal display device can make the afterimage almost unnoticeable by turning OFF the power sources of the liquid crystal display device and backlight concurrently, or bringing the liquid crystal display panel into an applied-voltageless state after the power source of the backlight is turned OFF. However, since the reflective liquid crystal display device can not block the incident light, it is impossible to make the afterimage less noticeable, thereby showing a display error vividly.
Further, the charges withheld in the liquid crystal due to an abnormal voltage not only degrade the display quality by the afterimage, but also give adverse effects on the operation life of the liquid crystal. In other words, the liquid crystal deteriorates, because, after the power source is turned OFF, the liquid crystal withholds the charges for as long as a few seconds until its potential drops to a GND level through natural discharge. In short, the liquid crystal deteriorates when an abnormal voltage is applied.
Japanese Laid-open Patent Application No. 170986/1989 (Tokukaihei 1-170986) discloses a method of erasing the afterimage, which is in effect an abnormal display occurred when the power source is turned OFF. According to this method, a power source maintaining circuit is provided to keep supplying an operating power to the liquid crystal panel for a predetermined period after the power source of the entire device is turned OFF, so that the active elements can stay ON for a certain period on a power supplier to a gate driver from the power source maintaining circuit, whereby the charges withheld in the liquid crystal display panel are discharged and the afterimage can be erased. FIG. 31 shows driving waveforms of the signals used in this method.
Incidentally, when a color image is displayed on the active matrix liquid crystal display panel, a voltage applied to the liquid crystal is controlled in a multi-level ranging from a threshold voltage to a saturation voltage. Here, the voltage applied to the liquid crystal and a response rate of the liquid crystal have a relation as illustrate in FIGS. 32(a) and 32(b). FIG. 32(a) shows a graph illustrating a relation between the number of levels and response rate in case of an 8-level display, and FIG. 32(b) shows a graph illustrating a relation among the number of levels, voltage, and transmittance. In FIG. 32(a), for example, “1-8” on the horizontal axis represents the level and means that the voltage is varied from the level 1 through level 8, where the level 8 represents a black display.
As can be understood from FIG. 32(a), the response rate of the liquid crystal varies with a level interval, and the response rate is slow particularly between the levels around the threshold voltage. This is because, when a voltage around the threshold voltage is applied to the liquid crystal, the distortion of the liquid crystal is so minor that only a small amount of energy is required to restore the liquid crystal.
Therefore, an amount of restoring energy is small in case there remains a half-tone afterimage around the threshold voltage when the power source of the liquid crystal display device is turned OFF. If such an afterimage is erased by the method disclosed in aforementioned Japanese Laid-open Patent Application No. 170986 (Tokukaihei 1-170986), that is, by releasing the charges withheld in the liquid crystal by maintaining the gate at an active level for a certain period after the power source is turned OFF, it takes a long time to release all the charges, thereby making it impossible to erase the afterimage quickly.
Moreover, merely maintaining the output of the gate driver at the active level does not reduce the potential of the liquid crystal panel to zero completely because of the operating conditions of the source driver, or the conditions of a voltage supplied to the liquid crystal from an opposing electrode in the liquid crystal display panel. Thus, in practice, a residual voltage is applied to the liquid crystal, and the above method presumably can not attain a desired afterimage erasing effect.
If the transmission liquid crystal display device adopts the above erasing method, still the afterimage appears after the power source is turned OFF; although the afterimage appears slightly for a short period, it is enough to degrade the image quality. Also, if it takes a long time to erase the afterimage, an abnormal voltage is applied to the liquid crystal due to the charges withheld therein even for a short period, and as a consequence, the liquid crystal deteriorates.
Further, the state of the reflective liquid crystal display device after the power source is turned OFF, and the state of the transmission liquid crystal display device with its backlight always kept turned ON are the same, meaning that the afterimage is more vivid in the reflective liquid crystal display device than in the transmission type. Thus, the liquid crystal deteriorates more or less the same extent in both the reflective and transmission liquid crystal display devices, but the display quality is deteriorated far worse in the reflective liquid crystal display device than in the transmission type.